In the marine environment herbivorous copepods represent an important link between the level of primary production and higher predators. In its main distribution areas of the North Atlantic and subarctic waters, the herbivorous copepod Calanus finmarchicus (Gunnerus, 1765) dominates the zooplankton communities in biomass. As a main prey species for commercially important fish stocks, C. finmarchicus links energy from the basis of marine pelagic food webs to higher trophic levels. C. finmarchicus performs diel vertical migration (DVM), highly synchronized to the diel fluctuations in light (day and night). The normal DVM pattern is characterized by an ascend to the surface at dusk and a descent to deeper layers at dawn. Yet, researchers have not found what the migration directly triggers. Light is supposed to be the most important cue. However, a few studies suggested that a biological clock is involved in DVM. The recent identification of clock genes in C. finmarchicus support the suggestion that an endogenous timing system may be involved in rhythms like DVM in C. finmarchicus. Thus, the aim of this work was to assess the role of light (photoperiod) on DVM and diel metabolic processes in C. finmarchicus and to detect the possible involvement of endogenous rhythmicity in these processes. To test this, laboratory experiments to the diel swimming behaviour and oxygen uptake rates were performed under light/dark (16 h L:8 h D) and constant darkness (DD) conditions, using the CV stage of Calanus finmarchicus. Copepods were sampled from an isolated population in Loch Etive, Scotland. In the laboratory experiments copepods showed a migration behaviour that is highly synchronized to the LD cycle, whereas a damped migration continued under DD conditions. Significant 24-hour oscillations in the vertical distribution were found in the migration experiment during the first day under LD conditions and during all three days under constant darkness. Also an oscillation in oxygen uptake rates was found under DD conditions. Overall, the results stress the importance of light for DVM of C. finmarchicus and suggest the involvement of an endogenous rhythm in diel patterns of vertical migration and metabolic processes. Regarding some limitations especially in respiration measurements, this work may provide a basis for further investigations on the true cause zooplankton DVM, implicating the role of biological clocks.